Several weeks later, following the death of King Robert during a boar hunt, Tommen’s brother Joffrey has succeeded to the Iron Throne despite reports of his false claim by the now-executed Lord Eddard, who threatened Cersei to inform the royal court of his discovery that her three children were not Robert’s children but born out of incest through her twin brother Ser Jaime. Outside of the Red Keep, young Tommen attends King Joffrey’s nameday tournament with his sister Myrcella. He witnesses his brother’s sadistic nature when torturing a knight named Ser Dontos Hollard, who arrives both late and intoxicated for his bout. Joffrey orders his guards to pour an entire barrel of wine down the knight’s throat with a funnel. When Joffrey orders Ser Dontos to be drowned with wine, Tommen shows visible discomfort. The king’s betrothed, Sansa Stark, however, convinces Joffrey to instead make Dontos his court fool, for which Dontos is grateful that she saved his life. Following this, Tyrion then arrives to the tourney, much to Joffrey's disdain, but to the delight of his nephew Tommen and niece Myrcella, who had worried that he was dead with regards to his long absence away from the city. Tyrion tells Tommen that one day he will be as big as Sandor Clegane, "but much better looking." With regards to the recent war between House Lannister and House Stark, as result of Lord Eddard’s execution, Tyrion then mockingly notes Joffrey’s absence from the battlefield, as the king indignantly responds that he has been ruling the kingdoms.

Some days later, Cersei hosts an awkward dinner with Tommen and Myrcella for the captive  Sansa Stark. Myrcella asks when Sansa and King Joffrey will wed and Cersei tells her that it will be after the war. Myrcella then remarks on the new dress she will have made for the day and then informs that Sansa's will be ivory as she is the bride. Cersei then prompts a silent Sansa to respond, as Sansa apologizes and assures Myrcella that her dress will be beautiful. Sansa then summons her usual line of forced devotion to King Joffrey and then asserts, as convincing a performance as she can muster, her “eagerness” for the war to finish so she can be wed to the king. Cersei, however, understands what it means to be in an arranged marriage and allows Sansa to continue peddling her dishonesty towards her son. Tommen then asks if Joffrey is going to kill Sansa's brother Robb Stark, the King in the North. Cersei asserts that Joffrey might and asks if Tommen wants him to. Tommen says that he does not, as Cersei insists that if Joffrey does Sansa will do her duty anyway. Sansa chokes down her helpless anger as her eyes fill with tears.

Weeks later, Prince Tommen and the royal court assemble at the shore of Blackwater Bay for the departure of his sister Myrcella, as she sets sail to Dorne to marry Prince Trystane Martell. A sobbing Tommen watches her departure from the harbor, as Joffrey mocks him for crying, though Sansa stands up for him. Meanwhile, Cersei is furious with Tyrion's arrangements for her daughter and urges that she hopes that he will truly love someone one day so that she can inflict the pain of taking them away from him. As the royal party walk back towards the Red Keep through the streets of the city, they are confronted by angry crowds of starving smallfolk. Sensing unrest, Tyrion orders Tommen to be sent along a separate route immediately, narrowly avoiding the Riot of King's Landing, which is triggered when someone throws excrement in Joffrey’s face. Tyrion is then horrified when the High Septon is then torn to pieces by the famished crowd. Once the royal family find safety, Tyrion confronts Joffrey for his part in the riot, calling him “a vicious idiot.” He slaps Joffrey when his nephew refuses to listen to the criticism. When they realize Sansa has been lost in the chaos, Joffrey callously commands that she be left to the mob. Tyrion has to point out that if any harm comes to Sansa then Ser Jaime will be killed in retaliation, who has been in the custody of House Stark for months. Sandor Clegane, however, eventually brings Sansa to safety, who was nearly raped by a handful of men.

Some time later, the late King Robert’s brother, Stannis Baratheon, and his fleet are hours away from attacking King’s Landing, due to the his rightful claim of the throne. Grand Maester  Pycelle is summoned to see Cersei in her chambers, who informs her that it is his duty not just to serve the royal house but also to offer guidance in times of conflict. Cersei impatiently asserts to him that his words are always wise and measured. She then begins to prompt him but he interrupts to say that a maester’s duties become more urgent in war time. He begins to reminisce and Cersei interrupts to ask if he has brought what she asked for. Pycelle confirms that he has and hands her a small bottle of Essence of Nightshade. He warns that it is as dangerous as it is efficacious and then explains that a single drop can cause relaxation, three drops a deep sleep and begins to explain what ten drops will do. Cersei interrupts him to say that she already knows. Pycelle asks permission for a question as Cersei curtly denies it. She says that he must be busy preparing for the battle. Cersei then warns him to take care as he descends the many stairs on his way out.

After Stannis’ arrival with his fleet the Battle of the Blackwater is underway. While King Joffrey and Tyrion lead the battle outside, Cersei provides refuge for the women and children of the court in a room within Maegor's Holdfast, a fortress inside of the Red Keep. With Tommen by her side, she prepares for the worst by having the executioner Ser Ilyn Payne present as well as the deadly poison from Pycelle. Cersei drinks wine heavily during the assault and seems to simultaneously torment and mother Sansa Stark with matter-of-fact descriptions of the rapes that will occur if the city falls, among other pessimistic musings. She then tells Sansa that the gods have no mercy and that Ser Ilyn is there to kill them should the Red Keep be breached. Later on, the wounded Lancel Lannister returns from the battle to report that Stannis Bartaheon has reached the Mud Gate, despite Tyrion's ingenious use of wildfire to destroy a large portion of his fleet. Cersei then commands Lancel to bring Joffrey back to the Red Keep, ignoring his protests that it will damage morale. It is later reported by Lancel that Joffrey willingly left the field but his exit is a blow to the discouraged Lannister men. Lancel then begs Cersei to let him return the king to the front, believing the battle is lost otherwise. Cersei pushes him into a stone pillar, aggravating his injury, and storms out with Tommen, leaving Sansa alone to deal with the frightened women who are increasingly losing hope of a win.

As the battle continues, a terrified Cersei sits on the Iron Throne holding Tommen on her knee. She offers to tell him a story in an effort to remain calm about a mother lion and a cub who lived in the woods. Cersei says that the mother loved her cub but that there were evil things like wolves that lived in the woods. She adds that the lions could hear them howling in the night and the cub was frightened but the lioness reminded her cub that he was a lion and told him not to be afraid. Cersei then says that one day all the beasts will bow to the lion cub and that he will be king. “The stags, the wolves, the bears in the north, the foxes in the south, the birds in the sky and the beasts in the sea will come to him to rest a crown upon his head.” As Cersei then uncaps the poisoned bottle, all hope lost, she tells Tommen that the lioness informed the cub that he would be strong and fierce just like his father. Cersei then puts the bottle in Tommen’s hand and promises to keep him safe. As Cersei is about to give the poison to her young son, Ser Loras Tyrell, whose house recently allied with House Lannister, bursts into the room with his soldiers. He rides in with Lord Tywin Lannister, Tommen’s grandfather who finally has returned to the capital along with his own army. Tywin pushes through the crowd and into the throne room, as Cersei pours the poison out and dropping the bottle to a shatter. He then announces that the battle is over and they have won. A relieved Cersei hugs Tommen close to her body.

Some years later, on the morning of King Joffrey and Lady Margaery Tyrell’s wedding, Tommen attends a breakfast celebration alongside his family. Margaery’s father, Lord Mace Tyrell, gifts Joffrey a magnificent golden chalice with seven facets, one for each of the major houses in the Seven Kingdoms and wishes him and his daughter Margaery a long and happy marriage. Joffrey graciously accepts. Queen Regent Cersei then spots Sansa Stark’s handmaiden named Shae and informs her father, Lord Tywin, that she is Tyrion's whore, as he then commands for her to be sent to the Tower of the Hand before the wedding. Tyrion overhears this and looks vexed, but proceeds to approach Joffrey and presents him with the “Lives of Four Kings,” a book detailing the reigns of four kings by Grand Maester Kaeth. Joffrey accepts the gift, but his sincerity seems questionable. A member of the Kingsguard then presents a Valyrian steel sword and Tywin tells him that it is one of two swords of its kind in the capital, as Joffrey excitedly rushes to wield it. He promptly begins to slash at Tyrion's gift despite its rarity.

Following the wedding ceremony, Tommen and the royal court gather outside the Red Keep for a lavish wedding feast in celebration of the union of House Lannister and House Tyrell. Tommen is sitting at the high table with the rest of the royal family when Joffrey speaks to the court about contemplating history, as five dwarves then ride out from a large lion’s mouth, representing Joffrey, Stannis Baratheon, Renly Baratheon, Robb Stark, and Balon Greyjoy, each of the five kings in the War of the Five Kings. Initially, Tommen laughs at his brother's insulting play, but stops and looks mortified when he sees Tyrion's face. Joffrey then publicly embarrasses his uncle Tyrion, however, after Tyrion outwits him in front of all his wedding guests, Joffrey petulantly pours wine over Tyrion's head and orders him to serve him wine as his cupbearer. After a few sips, Joffrey then begins to choke on his wine, causing him to gasp for air and collapse. Joffrey tries to speak, but begins coughing. He grabs his throat, as Queen Margaery exclaims that he is choking and needs help. After collapsing face down and vomiting, Tommen witnesses Cersei and Ser Jaime rushing in to turn Joffrey over on his back. Dontos Hollard, the drunken knight from Joffrey’s nameday tourney, suddenly approaches Sansa from behind and urges that she must come with him quickly, as they then flee the scene. Joffrey then dies in his mother’s arms, blood flowing from his eyes and mouth from the effects of the strangling poison. In a blind rage, Cersei accuses Tyrion and has him arrested.

Days later, following his uncle Tyrion’s arrest and Sansa’s successful escape from the capital, Tommen privately pays his respect to Joffrey in the Great Sept of Baelor, alongside his grieving mother. Lord Tywin enters and proceeds to quiz Tommen on the traits that make a good king, ignoring Cersei's protests and angry glares. Tommen suggests holiness, justice and strength as qualities of a good king. Tywin, however, refutes this by pointing out that kings Baelor Targaryen, Orys the First and Robert Baratheon, were each paragons of one of these qualities, but ultimately not very good kings. Tywin is delighted when Tommen, with minimal prompting, deduces that a good king must be wise. Tywin states to Tommen that young kings with no experience can become wise by listening to their councilors, and the wisest kings keep listening even when they don't need to anymore. After admitting that Joffrey was neither good nor wise, Tywin escorts Tommen out of the Great Sept and begins teaching him the duties of a king's marriage, as well as subsequent children. On their way out, Ser Jaime passes them and asks how his biological son is doing. The young soon-to-be-king, who remains unaware of the identity of his real father, replies that he is all right. Jaime assures Tommen that he will not let him meet the same end as his brother. In response to Joffrey's death, Cersei later insists to Jaime that Tommen be placed under high guard, with at least four Kingsguard at his bedroom door at night.

Some time later, Lady Olenna Tyrell informs her granddaughter, Queen Margaery, that she is about to leave for their home of Highgarden. In order to keep her title after Joffrey’s death, Olenna mentions the prospective marriage between Margaery and Tommen. She insists that Cersei will turn Tommen against Margaery by the time they marry if Margaery doesn't act fast. Olena notes that Cersei is distracted by Tyrion’s upcoming trial. Margaery states that Olenna cannot be certain of Tyrion's innocence, but Olenna assures her that Tyrion certainly is innocent and confesses that she would never have let Margaery marry "that beast," revealing that she was the one to slip poison from a necklace worn by an unknowing Sansa Stark into Joffrey’s goblet. Margaery is shocked, but her grandmother firmly ends the conversation. That evening, Margaery sneaks into Tommen's chambers, slipping past the Kingsguard and startling him awake. As she sits on his bed, Margaery suggests to him that they should get to know one another before they are to marry, something nobles in Westeros don't usually do. Tommen is unnerved by her presence until his cat, Ser Pounce, jumps up and Margaery interacts with him. Tommen sullenly reveals the cruel things Joffrey threatened to do to his cat, and he and Margaery bond over their mutual relief that they are now free of him. Tommen agrees that he'd like Margaery to visit him again. Before Margaery departs, she gives him a kiss on the forehead.

Days later, in the throne room of the Red Keep, Tommen is crowned King of the Seven Kingdoms and first of his name on the podium in front of the Iron Throne to enthusiastic applause. Following the ceremony Cersei meets with Margaery along the gallery of the hall. Margaery compliments Tommen by saying he sits on the throne as if he was born to it. Cersei enquires whether Margaery still mourns for the deceased Joffrey, which is met with a polite response. Cersei then admits to Margaery that Joffrey would have been her nightmare and goes on to comment on the things that he has done has shocked her. However, Cersei informs that no matter what they do you never love a child moreso than your first. She then ponders who the last decent king was and asserts that Tommen could be the first man in over fifty years that actually deserves to sit upon the throne. Margaery suggests that this could be consolation for all the horror that put him there. Cersei then requests Margaery’s help, as if to make a peaceful gesture towards her, saying that “a mother is not enough.” She asks Margaery if she is still interested in being queen, as Margaery coyly responds in saying that she will need to speak to her father about the matter. Margaery then snidely comments on the fact that soon she won’t know what to call Cersei, “sister or mother,” much to Cersei’s chagrin.

The following week, Tyrion is escorted into the throne room in front of the royal court to begin his trial for the accused murder of Joffrey. On his grandfather’s instruction, Tommen recuses himself from presiding over the trial, declaring Tywin, the Hand of the King and Protector of the Realm, as his replacement. Tommen then departs the throne room along with two Kingsguard. Several witnesses testify against Tyrion on the stand, such as Grand Maester Pycelle, who exhibits the poisoned necklace that Ser Dontos Hollard gave to the missing Sansa. Cersei herself then testifies, recalling a previous conversation she had with Tyrion where he threatened revenge on her, as well a noting Tyrion’s insistence of Joffrey fighting on the battlefield against Stannis’ army. After a recess, Tywin then orders a final witness to the stand: Shae, Tyrion’s former lover, who testifies that Tyrion and Sansa planned Joffrey’s murder together. With this, an enraged Tyrion explodes in front of the court and understanding that justice will not be served, demands a trial by combat. Some weeks later, Tommen is not in attendance during the trial by combat, which does not go Tyrion’s way as he is then sentenced to death. Tyrion, however, escapes from his cell shortly after and murder’s Lord Tywin before fleeing from the city.

Some time later, during Lord Tywin's wake, Cersei is approached by Ser Loras Tyrell, who babbles on with platitudes about her father, but she ignores him, as she is intently watching her son, King Tommen, sharing a few words with Margaery as they hold hands. Grand Maester Pycelle offers his condolences, yet she completely ignores him as well. Cersei is then approached by Lancel Lannister, who she previously indulged in an incestuous relationship with during the time her brother Jaime had been in imprisonment by House Stark. Lancel has become extremely pious over the years as he has joined the Sparrows, a religious movement that has risen from the Faith of the Seven. Cersei’s uncle Kevan Lannister, who has traveled from the Lannister seat of Casterly Rock to pay his respects to his brother Tywin, apologizes for the appearance of his son Lancel. Shortly thereafter, Cersei convenes the small council and announces that King Tommen has declared Qyburn the new Master of Whisperers and that the position of Hand will remain vacant until he is older and can decide for himself. Tommen has also decided to appoint Mace Tyrell as the new Master of Coin in addition to his duties as Master of Ships. He also has extended a newly-made position of Master of War to his granduncle Kevan, who exits the council stating that he will await Tommen's own instructions from Casterly Rock.

On the day of Tommen’s wedding, Cersei passes a large crowd on her way to the Great Sept of Baelor. She is increasingly annoyed as people cheer for Margaery, completely ignoring her own attendance. Inside the Sept, King Tommen then weds Queen Margaery in front of the royal court, much to the dismay of Cersei, who remains stone-faced throughout the ceremony. That night, following the ceremony, Tommen and Margaery consummate their marriage. Tommen is especially elated as the two discuss their future together, though he thinks that his mother is unhappy in King's Landing. Seeing an opportunity within that, Margaery manipulates Tommen into thinking that as long as Cersei stays in the capital, she will continue to treat him like a child, watching over him. The following morning, Tommen and Cersei are out for a walk along the battlements when he tells his mother that he thinks she should return to Casterly Rock if it will make her happy, since she used to talk so fondly of her childhood home. Cersei declines, however, deducing that this is Margaery’s doing. Cersei then pays her a visit, to find Margaery bragging about Tommen's libido to her handmaidens. Greeting her mother-in-law, Margaery subtly rubs her queenship in Cersei’s face, gloating about an “exhausting” night with her son. Cersei tries to verbally spar with Margaery as she used to, but the younger queen is in her prime, and assaults Cersei with backhanded compliments, including a comment on Cersei’s drinking habits. Instead of attempting to match the queen, Cersei walks away feeling defeated.

In the last few weeks, the extreme religious faction, the Sparrows, have risen in popularity and the Faith Militant that work beside them storm the streets of the capital, attacking taverns, brothels and street vendors selling idols from other religions. While this occurs, Lancel Lannister leads a squad of the Faith Militant to then apprehend Ser Loras while he is sparring in the practice yard. In her quarters, Margaery is furious over her brother’s arrest. Tommen then goes to his mother Cersei and demands that Loras be released at once, though Cersei calmly points out that she didn't arrest Loras, so he should ask their leader, the High Sparrow, to release him. Tommen naively thinks that the High Sparrow will simply do as he requests, so he heads to the Great Sept but his path is blocked by the Faith Militant, who refuse to move for the king. The Kingsguard request that Tommen let them kill the Faith Militant in order to pass. Tommen is surprised at this and does not want to risk bloodshed, so he meekly withdraws back to the Red Keep. Margaery is even more upset at how ineffectual young Tommen is and states that she is going to bring her grandmother Lady Olenna back to the capital to deal with the matter.

Some weeks later, the High Sparrow has called for a preliminary hearing to determine whether the charges of fornication and buggery against Ser Loras have merit, and expresses confidence that Loras will be acquitted. The High Sparrow himself interrogates Loras in front of King Tommen, Cersei and the Tyrell women. Loras denies the accusations against him and testifies that he has never laid with any man. Assuming the interrogation has concluded, Lady Olenna tries to leave with her grandchildren, however, the High Sparrow then asks to question Queen Margaery, who then also denies any knowledge of these allegations. The High Sparrow then calls in Olyvar, in character as Loras’ squire, who claims that the accusations against Loras are in fact true. Olyvar also testifies that Margaery walked in on them once in bed, but did not seem surprised. To support his testimony, Olyvar tells the High Sparrow of a birthmark Loras has that is shaped like Dorne, much to Loras' shock and fury. Deciding that this is more than enough evidence for a holy inquest, the High Sparrow has Loras arrested. Furthermore, since Queen Margaery bore false witness before the gods as well, she is also arrested. As Margaery is forcefully dragged away, she calls out for Tommen in panic, who he himself is paralyzed with indecision. Olenna then contemptuously regards Cersei's barely-contained smug expression.

Following his wife’s arrest and imprisonment, King Tommen has lost his appetite and rails over his inability to help Margaery. He voices his frustration as well as his plans to attack the Great Sept of Baelor in front of his mother. Cersei attempts to calm him down, however, the king wants to start a war for his queen, declaring his love for her to a dumbstruck Cersei. She gives him a long speech about how as king he will be faced with many circumstantial situations in which he will be powerless to act, as Robert Baratheon sometimes was. Cersei then proclaims her love for her son by claiming that she will do anything it takes to protect him, even if it means burning down entire cities to the ground. Cersei then weeps as she holds a distraught Tommen in her arms. To maintain control of the situation, she offers to speak with the High Sparrow for on his behalf and advocate for Margaery and Ser Loras' release. Although clearly suspicious, the still-overwhelmed Tommen agrees.

Afterwards, in the cells of the Great Sept, Cersei meets with Margaery and brings her food but the queen refuses, revealing that she knows her mother-in-law is responsible for having her and Loras imprisoned. Cersei, fearful of being overheard, denies this. Margaery mocks Cersei for her unconvincing lies of trying to save her, going as far as to say that while Cersei may know how to lie and scheme, she doesn't understand innocence and decency well enough to fake them. Margaery then claims that perhaps this is the reason her son Tommen was willing to cast Cersei aside and send her back to Casterly Rock. Cersei, concealing her fury, tells Margaery her trial will soon begin and that her son needs his mother more than ever. Margaery throws her stew at Cersei, yelling: “Get out, you hateful bitch!” Cersei leaves the cell, smiling smugly at Margaery. On her way out, Cersei thanks the High Sparrow for his arrests, as he proceeds to give her a speech about the history of the Great Sept. He informs her that the Tyrells' falsehoods will be revealed for all the world to see, as with the case for everyone, including Cersei herself. The High Sparrow then asks her: “What will we find when we strip away your finery?” He then reveals that her cousin Lancel confessed his incestuous affair with Cersei, and orders for her arrest, much to her surprise and horror. She is taken to a cell by a group of Septas, and after she is thrown onto the floor, Cersei threatens them that her face is the last thing they will see before they die.

Cersei remains in custody for the next few weeks, visited only by Septa Unella, who offers her water in return for a confession, but she repeatedly refuses. Cersei is later visited by Qyburn, who informs her that the charges against her are treason, incest and regicide, the latter charge for organizing the assassination of King Robert with Ser Lancal. With conviction, Cersei dismisses these charges as lies. Qyburn warns her that the Faith will not depend on physical proof as much as the crown does in her upcoming trial. Qyburn then also informs that there has been no response from Jaime with regards to extracting their daughter Myrcella from Dorne. Qyburn further informs Cersei that Pycelle has summoned her uncle Kevan back to the capital and King Tommen has now fallen into a depression. Qyburn, however, asserts to Cersei that she can still confess, however, she stubbornly refuses. Afterwards, when Unella visits her again, Cersei offers to make her a Lady of the capital or have her killed, depending on whether or not she will help her. In response, Unella pours her water on the floor and leaves. Cersei finally abandons her pride and licks the water from the dungeon floor, breaking into tears. Some days later, Cersei is then brought outside on the steps of the Great Sept and forced to strip down naked in front of the entire city and walk towards the Red Keep. Though Cersei is then released from imprisonment following her walk of atonement, the High Sparrow still plans on organizing a trial for her charges in the near future, while Tommen increasingly distances himself from his mother.

Some time later, Ser Jaime returns from Dorne with the corpse of Tommen’s deceased sister Myrcella, who was poisoned by Ellaria Sand, a Dornish woman who is working against House Lannister. The funeral is held at the Great Sept but Cersei is not allowed to attend, as King Tommen himself has ordered for Cersei to be confined in the Red Keep since he is concerned that the Faith Militant will arrest her again. Inside the Sept, Tommen and Jaime discuss Myrcella, as well as the also-murdered Dornish prince, Trystane Martell, who was recently assassinated in the capital. The young king incorrectly believes that Cersei is to blame for Trystane's death but insists to Jaime that he isn't upset with her. Jaime chastises Tommen for this very action, as he himself is irate over his sister not being able to attend their daughter’s funeral. They are interrupted by the High Sparrow and after Tommen departs, he then visits Cersei in the Red Keep and apologizes for not being more assertive during her imprisonment. He regrets letting the High Sparrow lock her away and asserts that he should have torn the Sept down with the High Sparrow inside of it. Tommen tells her that he needs her help in being a strong ruler, especially in light of his earlier humiliation with the Faith Militant. Cersei readily accepts his apology and offers her son help in gaining his desired power and effectiveness.

Days later, Tommen and his Kingsguard visit the High Sparrow and demands for his mother Cersei to visit Myrcella's crypt in the Great Sept of Baelor. The High Sparrow denies Tommen's request on the grounds that his mother has not been cleared of the other crimes, namely killing King Robert, as well as her charge of incest with Ser Lancel. Tommen then criticizes the Sparrow for his harsh and self-righteous treatment of his mother when she has already performed her walk of atonement in relation to her other two crimes: falsehood and fornication. After stating his demand again, Tommen asserts his right as king and asks the Sparrow what that means to him. The religious leader states that it means a great deal, as the Faith and the Crown are the two pillars of the world. Despite their differences, the High Sparrow then claims admiration for the deep love that Cersei has for her son, which he himself attributes to the divine love of The Mother, one of the seven manifestations within the Faith of the Seven religion. When Tommen questions why the High Sparrow wants to punish Cersei, the latter replies that he is merely carrying out the will of the Seven. The High Sparrow then urges Tommen to turn to the Seven for wisdom. Before Tommen leaves, less angry and more uncertain of right and wrong, the High Sparrow tells him that the Seven have worked through his grandfather Tywin and his mother Cersei, even though they may not acknowledge it.

Some time later, Cersei goes to speak to Tommen, who is anxious to fight the High Sparrow. He then proposes that they shouldn’t antagonize the High Sparrow in order to safeguard Margaery. Cersei, however, reminds him of what the pious man forced her to do during the walk of atonement. Although Tommen knows his mother has always hated Margaery, Cersei claims the rivalry is unimportant, adding that the Sparrow has no respect for kings, queens and society, and the High Sparrow is little more than an idealistic anarchist using faith to achieve his goals. After Tommen reveals his conversation with the High Sparrow and knowledge of something important, as Cersei presses him for information. Afterwards, Cersei and Jaime then attend a small council meeting, presided over by Kevan Lannister and Olenna Tyrell. Cersei reveals that Tommen has been talking to the High Sparrow about Margaery and Loras, and the former will have to perform her own walk of atonement, which Lady Olenna agrees must not happen. Although Cersei promises to destroy the Sparrows for corrupting his son Lancel, Kevan warns them that more lives could be lost in the ensuing battle. Olenna then urges that many will die regardless and “better them than us.”

Some weeks later, inside the Great Sept, King Tommen is speaks with the High Sparrow about Queen Margaery’s impending walk of atonement and his fear that it could be dangerous. Understanding Tommen’s concern the High Sparrow insists the queen will be protected the entire walk, stating that the Faith Militant are very stern with those who overstep their bounds. The High Sparrow then offers to let Tommen visit with Margaery, as the Sparrow trusts that she too is now devoted to the Faith of the Seven. As promised, Tommen then meets with Margaery and is surprised to find her speaking positively about the High Sparrow. He is also surprised to hear her speaking contritely about her past sins, including her vanity about wanting to be seen as good person with regards to her charitable actions for the poor and needy. Tommen eventually finds himself agreeing with her feelings towards the High Sparrow, as Margaery then asserts that there is something about the devoted man and his way of viewing the world. When Tommen asks of her brother, Ser Loras, and his imprisonment, Margaery explains that she loves her brother, who is a “pure soul.” However, she notes that his sins don’t erase his purity, it only obscures it. Margaery then admits that Loras needs to atone for his sins, as does everyone.

Shortly after, Mace Tyrell leads an army through the streets of King's Landing and meets with Ser Jaime. Together, they proceed to the steps of the Great Sept, where the High Sparrow, the Faith Militant and a large crowd of city folk are preparing for Margaery's walk of atonement. The army arrives, along with Lady Olenna in a litter, as the High Sparrow is speaking, and Jaime demands he release Margaery and Loras, before they can "be on their way." The High Sparrow refuses and fends off Jaime's threats to kill all the Sparrows by saying each Sparrow yearns to die in the service of the gods. Alas, he proclaims that it will not be necessary, as there will be no walk of atonement, Margaery has already atoned for her sins by bringing another into the faith. To Jaime's astonishment, the doors of the Sept open and King Tommen emerges and walks down the steps to join the High Sparrow and Margaery in a demonstration of unity between the crown and the faith. The crowd burst into cheers after Tommen's speech, demonstrating that the High Sparrow now has the support of both the crown and the smallfolk. Baffled, Lord Mace asks what is happening, as Lady Olenna replies in disgust that the High Sparrow has beaten them. At the same time, a furious Jaime seethes at the High Sparrow's triumphant expression. Afterwards, in the throne room, Tommen strips Jaime of his position as Lord Commander of the Kingsguard because "an attack on the faith is an attack on the crown." Tommen then places Jaime in charge of a Lannister army that is being sent to assist House Frey lay siege to Riverrun.

Some time later, Cersei and Qyburn enter the throne room to find a large crowd as there is a royal announcement. Cersei asks her uncle Kevan why she was not informed, but Kevan bars her from standing beside King Tommen, stating that her place is in the gallery "with the other ladies of the court." After Cersei joins the ladies in the gallery, Tommen starts by saying that the crown and the faith are the two pillars that hold up this world, and should one collapse, so does the other. He also tells the court that The Father judges them all, and if they break his laws, they shall be punished. He announces that Ser Loras and his mother’s trial will be held on the first day of the ‘Festival of The Mother’. After much prayer and reflection, Tommen also announces his decision that trial by combat will now be forbidden in the Seven Kingdoms, calling it a scheme made by those who want to escape true judgement from the gods, and that Loras and Cersei will stand trial before seven septons as it was in the earlier days of the Faith. As the king exits, Qyburn tells a shocked Cersei, who had been planning on calling for a trial by combat with Ser Gregor as her champion, that his little birds have been investigating an old rumor that she had told him about, and that it was found to be much more than a rumor.

Some weeks later, everyone is getting ready for the trials of Loras and Cersei. The crowd amasses inside the Great Sept, alongside Queen Margaery, Lord Mace and Kevan Lannister. As Grand Maester Pycelle heads to the Sept, a small child stops him and whispers something into his ear that suddenly changes his course. In the Sept, the Sparrows bring Ser Loras in first, as the High Sparrow awaits. Instead of having a trial, however, Loras quickly confesses to his crimes and tells the High Sparrow that he wishes to devote his life to the Faith of the Seven. Margaery and Lord Mace watch solemnly as the High Sparrow informs Loras that this would mean that he renounces his family name and his future lordship of Highgarden, as Loras tearfully agrees. The Sparrows then grab him, hold him down and carve a seven-pointed star into his forehead. His father Mace actually tries to fight his way through the crowd to stop them but Margaery, her voice trembling, holds him back. Shocked by this action, Margaery afterwards quietly protests to the High Sparrow for mutilating her brother. Ignoring this, he then informs Margaery of his promise to allow Loras to leave once Cersei has faced her trial as well. Meanwhile, King Tommen prepares to depart from his chambers in the Red Keep for his mother’s trial, but Ser Gregor Clegane prevents him from leaving his room. In the Sept, Lancel Lannister tells the High Sparrow that Cersei has yet to leave the Red Keep, so the High Sparrow asks him to go retrieve her. As Lancel is walking out, another small child lures him away to the catacombs beneath the Great Sept. Meanwhile, Grand Maester Pycelle follows the whispering child into Qyburn’s laboratory, where Qyburn himself awaits his arrival and revealing the children to be his aforementioned “little birds”. Qyburn apologizes to Pycelle for the inconvenience, then multiple children surround the grand maester and begin stabbing him repeatedly to death. Below the Sept, Lancel continues to follow the young boy, who drops the torch that he was carrying. When Lancel goes to pick up the torch, the child stabs him in the spine and runs off. Paralyzed from the waist down, Lancel looks around where he is laying and discovers that there is a cache of wildfire positioned in the catacombs, once placed there by the “Mad King” Aerys Targaryen as part of his plan to deny the city to his enemies when facing certain defeat at the end of Robert's Rebellion. As Lancel crawls closer to the main source of the illuminated green liquid, he sees that the wildfire is about to be set off with three candles at the end of the hall that are almost burnt out. He desperately crawls toward it in an attempt to put the candles out. Meanwhile, an impatient and increasingly worried Queen Margaery realizes that something is very wrong as they continue to wait for Cersei and Tommen’s arrival. She immediately confronts the High Sparrow, insisting to him that this is a trap, frustratedly informing him that “Cersei understands the consequences of her absence and she is absent anyway, which means she does not intend to suffer those consequences.” She demands for everyone to exit the Sept, however, the High Sparrow does not listen to her astute assessment of the situation. Margaery then tells everyone in the Sept that they all need to leave immediately, however, the Faith Militant block the doors, preventing anyone from exiting. Down below, Lancel does not make it to the candles before the wildfire then ignites, flooding the tunnels with green flames and claiming him as the first casualty. In the Sept, Margaery and the High Sparrow share a look of terror as the gathered nobles hear the muffled explosions before the fire then punches through the floor, burning the High Sparrow down to his bones where he stands, along with Margaery, Loras, Mace, Kevan and the rest of the royal court in attendance. The wildfire burns with such intensity that the Great Sept then shatters and crumbles to rubble, flinging debris far and wide, leveling the city for a mile around. Cersei, looking out one of the windows in her room, smiles at the explosion even as the screams of the dying population ring through the air. Meanwhile, from his own chambers, Tommen stares at the explosion in shock and disbelief.

After being informed of the explosion and its casualties, including his beloved wife, Queen Margaery, one of King Tommen’s servants continues to offer his condolences to the young monarch. He continues to stare out the window, still in a state of catatonic shock, trying to rationlize his knowledge of the fact that his own mother was responsible for the death of his queen, along with many innocent lives. After the servant exits his chambers, Tommen’s stare turns into a determination, as he then removes the crown off of his head and places it down on a nearby table, walking away from the gruesome view of the smoky Sept in the distance. He then quickly returns to the horrific scene through the window and with conviction Tommen calmly steps up to the window frame. Without hesitation, he then falls out of the window to his death. Some time after Tommen's body is found on the ground of the Red Keep, his mother’s triumph is greatly diminished at the devastating loss of her last child. Standing by his covered remains in a dark room with Qyburn, Cersei insists on seeing her son's face one last time. Qyburn then asks Cersei what she wants to do about funeral arrangements, since the Great Sept is obviously no longer an option. Cersei tells him to burn Tommen’s body and bury the ashes where the Sept once stood, so that her son may rest with his brother Joffrey, his sister Myrcella and his grandfather Tywin.

Later that afternoon, returning from his mission at Riverrun, Ser Jaime and the Lannister army arrive on the outskirts at King's Landing. Jaime’s face that is full of triumph instantly turns to one of shock as he witnesses smoke still pouring from where the Great Sept and its surrounding districts once stood. Once returning to the Red Keep, Jaime enters the throne room, which is in full attendance, in time to see Cersei and her Kingsguard enter and walk the path towards the Iron Throne, flanked by Lannister guards. Cersei ascends the steps and stands before the throne. Qyburn proclaims Cersei Lannister as the Queen of the Seven Kingdoms and all the associated titles. He places a crown on her head and she sits down on the Iron Throne. The crowd of nobles glare at her with undisguised hatred, aware of what she has done in order to gain power, seemingly displaying loyalty only out of fear of Cersei's wrath. Cersei ignores the nobles as she has eyes only for her brother Jaime, who stares back at her with a stony expression. With a steely gaze, she looks out towards the room as the new queen, taking in her ultimate ambition that has come to life: the ruler of Westeros- or at least, what little of it the Lannisters can still control.

*In the aftermath of Tommen’s death, follow Cersei's journey throughout the rest of the series.